1. Field of the Invention
The present invention is directed to the field of power supplies and is more specifically directed to a power supply suitable for use as a flash lamp simmer supply.
2. Background
Flash lamps, which are generally filled with xenon or krypton, produce intense pulses of light when subjected to an increase in voltage from below to above the voltage required to generate an arc through the lamp. The lamps are not operated in a continuous fashion, but, as indicated by their name, in a flashing mode by supplying pulsed voltage to the lamps.
Because of the intensity of light produced, such lamps are desirable for high intensity applications; however, such lamps have previously not been operated with high efficiency when used to produce ultraviolet light. Operation of a flash lamp with high output in the deep ultraviolet requires very high peak power and current levels, thus forcing the lamp to operate under conditions not required for normal visible light production.
For example, it is generally desirable to operate flash lamps in a so-called simmer mode, in which a small current is passed through the lamps on a continuous basis, with the simmer current and voltage being insufficient to produce the high-energy flash of light. When flash lamps are operated in the simmer mode, they typically have increased lamp life and improved efficiency of conversion of electrical energy to light energy. Operation in a simmer mode also eliminates the need to restart the lamp prior to each flash.
However, the high voltage and amperage required for efficient UV light production causes problems with existing simmer circuits. The major drawback to use of flash lamps in a simmer mode in both visible and UV light production is the relatively large amount of power consumed by the simmer power supply. The classic simmer supply is a high-voltage power supply connected to the lamp through a string of resistors referred to as "ballast" resistors. Such a power supply produces a relatively constant current through the lamp, because the relatively high resistance of the ballast resistors provides most of the resistance in the circuit despite variations in voltage across the lamp during discharge. The particular voltage used will depend on the operating characteristics of the lamp being used, but typically is in the range of several hundred to more than a thousand volts (e.g., typically about 1500 volts for a six-inch arc). The high open-circuit voltage from the simmer power supply has the additional advantage of reducing the energy required to start the lamp.
A major disadvantage in such a system is that the ballast resistors dissipate significant power. For example, if a lamp is to be simmered using a 1500-volt power supply at a current of 1 amp, and the lamp voltage drop at that current is 100 V, then the power dissipated by the resistors is 1400 W while the lamp energy dissipation is only 100 W. If a lamp were to be operated at a pulse input power of 4,000 W (average), then the simmer power would represent 37.5% of the total lamp power consumption.
An apparent technique for overcoming this difficulty would be to use a switching power supply to provide the 1 amp of simmer current as needed by the circuit. Switching power supplies are well known and can be purchased through commercial electrical suppliers. If a switching power supply with an efficiency of 80% were used, then the total power consumed by the simmer current would be 125 W, or only 3.1% of the lamp pulse power. Such a system has not been amenable to prior design, however, since the lamp, when pulsed, has a dynamic impedance that changes its V/I characteristics on the millisecond time scale following a flash. The simmer current must be maintained at a constant nominal value despite this variation in lamp impedance. Typically, at a 2 amp simmer current, the initial voltage drop as measured with a resistive power supply is approximately 120 V, increasing to 200 V during operation before returning to the steady-state value. If the simmer current and power supply voltage are not properly matched, the lamp voltage drop can reach a peak at which the supply can no longer provide sufficient current. If this happens, the lamp will go out, and the advantages of using a simmer mode will be lost.
Prior simmer circuits used with relatively low-power flash lamps operating in the visible light range overcame this problem by including the previously mentioned ballast resistor, with the resulting loss in efficiency as discussed above. This was not a problem in operations such as flash photography, where energy consumption is not normally a relevant issue. However in UV applications involving chemical photolysis, such as are being contemplated by the present inventors, continuous flashing at high voltage is required, and the inefficiency of existing simmer circuits represents a major cost in the operation of the lamp.
Circuits have been designed for use in other applications where the current needs to remain constant in spite of variations in load resistance. For an example of such a circuit used for other applications, see U.S. Pat. No. 4,748,551. However, such circuits are not directly applicable to flash lamp power supplies, as they provide time-averaged (rather than instantaneous) constant current through the varying resistance and are designed for AC (rather than DC) operation. Accordingly, there remains a need for switching power supplies useful as an ultraviolet flash lamp supply.